Petroleum refiners have a continuing need for improved methods for removing components from their products, which have the potential of being harmful to the environment. Fuel products, such as diesel fuels, or middle distillates, are representative of those products which must meet ever increasing governmental restrictions with respect to the level of heteroatoms, such as sulfur and nitrogen components. For example, feedstocks for producing diesel fuels will typically contain about 1 to 2 wt. % or more sulfur, but the final fuel product, to meet governmental requirements, cannot have more than about 0.05 wt. %, and in the not too distant future, no more than about 0.005 wt. % sulfur is likely.
While high severity processing can reduce the level of sulfur and nitrogen components to 0.05 wt. % or less, the high temperatures used in such processing produces a fuel with poor color qualities. This is primarily because undesirable reactions, such as condensation and dehydrogenation reactions occur at high temperatures, resulting in components which are dark in color and which are extremely difficult to remove.
Much work is being done to develop more active catalysts and improved process designs to meet the demand for middle distillates which are low in sulfur and nitrogen, but which still meet the ASTM color requirements for the final fuel products. Various process scenarios have been suggested to reduce the level of heteroatoms in middle distillates, with or without maintaining good color quality. For example, Japanese Patent Laid-Open Application No.3-86793 teaches a two-step hydrotreating process for the production of a diesel gas oil having a sulfur content of 0.2 wt. % by weight or lower. The first step requires a temperature in the range of about 280.degree. C. to about 370.degree. C. and pressures from about 10 to 40 kg/cm.sup.2. The second step requires a temperature in the range of about 150.degree. C. to about 325.degree. C. and similar pressures as the first step. Such a two-step process has difficulty producing products that are low enough in sulfur and which meet acceptable color specifications.
Also, U.S. Pat. No. 4,755,280 teaches a two-step hydrotreating process for improving the color and oxidation stability of hydrocarbon compounds wherein an Fe-type catalyst is employed in the second step. However, it is known that the hydrotreating activity of Fe-type catalysts is easily poisoned with hydrogen sulfide. Therefore, the amount of sulfur and nitrogen compounds present in the feedstock to be fed to the second step must be lowered to such uneconomical levels as 10 wppm.
Further, U.S. Pat. No. 3,841,995 proposes a two-step hydrotreating process for the improvement of the color and odor of hydrocarbon compounds. However, in the process a noble metal catalyst, such as a Pt catalyst, is used in the second step, thus, the hydrotreating activity of the catalyst is readily poisoned by sulfur components.
European Pat. Application 0523679 A2 teaches a two-step process for producing low-sulfur diesel gas oil with good color properties. The first step is conducted at a temperature from about 350.degree. C. to about 450.degree. C., and a pressure of about 45 to 100 kg/cm.sup.2 in the presence of a hydrotreating catalyst and a the second step is conducted at a temperature from about 200.degree. C. to about 300.degree. C. and a pressure of about 45 to 100 kg/cm.sup.2.
Other process configurations include the use of multiple reaction stages, either in a single reaction vessel, or in separate reaction vessels. More sulfur sensitive catalysts can be used in downstream stages as the level of heteroatom components becomes successively lower. European Pat. Application 93200165.4 teaches a two-stage hydrotreating process performed in a single reaction vessel, but there is no suggestion of a unique stripping arrangement for the liquid reaction stream from each reaction zone, and there is no suggestion of quenching the effluents from the first stage prior to hydroprocessing in the second stage.
While there is a substantial amount of art relating to reducing the heteroatom content of distillate feedstocks, there still remains a need in the art for improved processes that can produce middle distillates low in heteroatoms and yet within acceptable color specifications.